Further Study of the Atomic Weight of Lead of Radioactive Origin (1916)(en)(7s)

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694

CHEMISTRY: RICHARDS AND WADSWORTH

ton, as represented by changes in the total ash of the body, which pro-

ceeded for 4 time at the expense of other tissues while the animal was held at constant weight, does not now, in the active growth which accompanies refeeding, keep pace with this rapid gain in body weight, and consequently in a few days the normal relation is almost reestablished.

FURTHER STUDY OF THE ATOMIC WEIGHT OF LEAD OF RADIOACTIVE .ORIGIN By Theodore W. Richards and Charles Wadsworth, 3d WOLCOTT GIBBS MEMORIAL LABORATORY, HARVARD UNIVERSITY Read before te Acade.'Noveber 14. 1916. Reved, November 27. 1916

The recent independent and almost simultaneous investigations upon the atomic weight of lead from radioactive minerals have proved with very little room for doubt that the substance derived from this source has a much lower atomic weight than ordinary lead.1 This conclusion is so important in its theoretical relations that its every aspect should be carefully investigated. Accordingly, the present paper represents further research in this direction, embodying determinations of the atomic weight of new samples of varied origin. The outcome entirely supports the earlier conclusion. Four samples from widely separated sources were studied in the present research, namely, lead from Australian carnotite, from American carnotite, from Norwegian cleveite, and Norwegian br6ggerite. The first of these samples was obtained in large quantity through the kindness of Mr. S. Radcliff and Mr. E. R. Bubb, of New South Wales. The preliminary purification of the sample was carried out in Australia.2 Our subsequent purification was briefly described in a recent paper,8 but some additions to the account are needed. The metallic lead was dissolved in nitric acid, leaving practically no residue. A portion of the nitrate thus obtained was precipitated with 20% hydrochloric acid from dilute solution. Lead sulphide was precipitated from the warm acidified solution of this chloride by pure hydrogen sulphide, and after separation and washing was dissolved in pure nitric acid. The small portion oxidized to sulphate during this process was boiled with sodium carbonate and the lead carbonate, washed free from sodium, was dissolved in nitric acid and united to the main portion of the nitrate, which was recrystallized four times from pure water and precipitated as chloride from a warm solution in a quartz

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695

dish with hydrogen chloride. The precipitate was centrifuged, dissolved in water and recrystallized four times, each time having added a few drops of hydrochloric acid to prevent the formation of basic salt. The second mother liquid gave no test for nitrate. This chloride formed Sample A, and after two additional recrystallizations a similar specimen constituted Sample B. The second portion of the original nitrate made from the Australian sample was purified in a much simpler fashion. Avoiding the troublesome precipitation with hydrogen sulphide, the nitrate was recrystallized five times successively by adding concentrated nitric acid to its aqueous solution; then the lead was converted into chloride, and by precipitation with excess of hydrochloric acid this salt also was recrystallized five times. This was Sample C of the chloride, a portion of which before the last crystallization had served to prepare Sample D of the metallic isotopic lead used for determining the density.4 The next sample, designated F, was prepared from American carnotite and came to us in a considerably purified state through the kindness of Dr. C. H. Viol of Pittsburgh, of this company, and of Prof. W. D. Harkins. The method of further purification was essentially like that just described.6 Two other samples of especial value and significance were obtained through the kindness of Dr. Ellen Gleditsch, of Kristiania. Both came from primary rocks-Norwegian pegmatite dykes. The purification of one of these, from cleveite, has already been described;6 the source of this material, which occurred in cubic crystals and was carefully selected, was near Langesund, Norway. It was recrystallized first as nitrate and then as chloride three times, each in the usual manner, and the pure substance was designated as Sample G. Yet another sample, designated H, was prepared from lead sulphide, also kindly sent by Dr. Gleditsch, obtained from selected crystals of Norwegian octahedral br6ggerite from Roade, near Moss, Norway. This was purified in precisely the same way as lead from cleveite. In addition to these four samples containing isotopic lead enough ordinary lead was carefully purified to serve as the basis of control analyses. The purest "test lead" of commerce, free copper, was dissolved in nitric acid and recrystallized four times as nitrate and four timn as chloride. Throughout this work the usual care taken in atomic weight investigations was not forgotten. The nitric acid, hydrochloric acid, water and silver were all purified in methods already often described, and throughout the work on the nitrate and chloride of lead, except

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in the preparation of Sample A, the material was treated exclusively in vessels of platinum or quartz. All the weighings were reduced to the vacuum standard and all other precautions usual in this sort of work were carefully maintained. The analysis was essentially similar in every way to the method described so often in Harvard contributions. The lead chloride was fused in a platinum boat in pure hydrochloric acid; this gas was displaced with nitrogen while the substance was cooling; and finally the pure dry salt in its boat was pushed into the weighing bottle, stoppered in pure dry air with the help of the familiar Harvard "bottling apparatus" and weighed at leisure. The weighed salt was placed in a large Erlenmeyer flask with glass stopper very carefully ground. Enough water was then added to form a fiftieth normal solution of the salt and the flask and contents, with the addition of a drop of pure nitric acid to prevent the formation of basic salt, were gently warmed on an electric stove, at about 50°C., until complete solution was obtained. Theboat was then removed, and the residue filtered off, both boat and residue being carefully washed and the filtrate being collected directly in the precipitating flask. The chlorine contained in this solution was then precipitated in the usual fashion by an amount of silver calculated from preliminary trials to correspond with it as nearly as possible. Any slight deficiency or excess was corrected by adding silver nitrate or chloride, and testing in the nephelometer; and the finally corrected weight is given in the table. The precipitation was carried on in a dark room, under red light, and all the usual precautions were taken. The first two determinations of ordinary lead were only preliminary, in order to gain practice with the method, and are not included in the table below. They yielded values for the atomic weight respectively 207.15 and 207.16. All the other analyses which were brought forward to conclusion are recorded in the tables, which are self-

explanatory.

~~-T

The atomic weight of ordinary lead

BSAMPB

3 4

wrAmg PbC1 WIGTPbCh sAEIRCOR.ECIED ADD 3.72918 5.35111

2.89325 4.15151

ATIO

ATOMIC

P :Ag. PbCl

wIGT Pb

1.28892 1.28896

207.179 207.188

Average

207.183

CHEMISTRY: RICHARDS AND WADSWORTH The atomic

wight of "isotopic" lad

COIRRECTED PbCl 6 8wuoEr

OREC Dr Ag

Carnotite, Australia....... Carotite, Australia....... Carnotite, Australia....... Carnotite, Australia.......

4.64010 5.35517 6.15608

3.61118 4.16711 4.79072

4.14770

3.22748

5 F Camotite, U.S.A.........

5.31585 4.65899

4.12670 3.61707

7 H Br3ggerite, Norway........ 8 G Cleveite, Norway.......... 9 G Cleveite, Norway..........

4.29104

3.34187 3.05913 3.46818

sau~

1 2 3 4

A B B C

6 F

Carnotite, U.S.A.........

3.92736

4.45270

'

UATIO

PbCh: Ag 1.28493 1.28512 1.28500 1.28512

Average

analyses

Average

1.28402 1.28382

1.28387

show that the different

ATOMIC

wPa

Pb

206.318 206.359 206.334

206.359 206.342

1.28816 1.28806

Average

The results of these

697

207.015 206.994

207.004 206.122

206.079

206.090 206.084

samples

con-

taining isotopic lead all give lower values for the atomic weights than

ordinary lead, but that the material from each source gives a different value, precisely as had been previously found in the earlier investigations in this and other laboratories. Ordinary lead gave the maximum value (essentially equal to that found by Baxter7) and isotopic lead from Norwegian cleveite gave the minimum value (essentially equal to that found by Honigschmid in broiggerite8 206.06). It seems reasonable to suppose that the other samples were composed of mixtures of these two kinds of lead. The value 206.34 would be given approximately by a mixture of three parts of isotopic lead like that obtained from Norwegian cleveite with one part of ordinary leada reasonable supposition, since the Australian carnotite was known to have contained galena. The American carnotite, Sample F, had an atomic weight which would be given by a mixture of only one part of the pure isotope with 5 or 6 of ordinary lead,-a condition which. seems to indicate the admixture of very large amounts of galena with the sample-in question. Two physical properties of the several preparations under consideration have especial interest, namely, the magnitude of the radioactivity and the nature of the spectrum. Both were studied in the present research. We confirmed the outcome of earlier work' that the radioactivity is not proportional to the decrease in atomic weight in samples containing isotope coming from different sources. If the radioactivity were dependent upon the presence of the isotope radium

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CHEMISTRY: RICHARDS AND WADSWORTH

G (the supposed-end product of the decomposition, the present form of isotopic lead), the rate of fall in the gold leaf electroscope for the lead from cleveite would be the greatest, and that from the American carnotite the least; but our results were precisely the other way about. There can be little question, then, that radioactivity is not due to the isotope which gives the low atomic weight. Probably it is due to radium E, since in all of our samples the half-value of the radioactivity was obtained in about five days, the half life of radium E. The maximum value, approached asymtotically, is nearly reached in a month. The spectrum of isotopic lead as thus far studied by H6nigschmid, by Merton and by Harkins, as well as by Baxter (who kindly photographed the ultraviolet spectrum of one of the samples of lead prepared by one of us with the help of Dr. Lembert) has always been found to be essentially like that of ordinary lead. In the present research we thought it worth while once more to test this question and especially to extend the inquiry to the visible portion of the spectrum-most of the tests in the past having been made in the ultraviolet region. We found difficulty in eliminating traces of copper, silver and calcium too small to be detected by ordinary analysis. Although these were far too diminutive to affect the atomic weight, we continued the purification, and after eight recrystallizations as nitrate and two as chloride, the product gave in every respect precisely the same spectrum as the purest ordinary lead prepared by Baxter and Grover, except for a vanishingly small trace of two of the most prominent silver lines. The photographs of the ultraviolet region in the F6ry spectrometer were very kindly made by Professor Baxter. Photographs of the visible portion of the spectrum were made by us in the Gibbs Laboratory on specially prepared plates sensitive as far as wavelength 7800. All the work of loading and developing had to be done in complete darkness. To make assurance doubly sure, a further study of the visible spectrum was made, in collaboration with Mr. Norris F. Hall, with the help of the Hilger wave-length spectrometer, comparing visually in the same field of view the spectra of pure ordinary lead and the best purified specimen from Australian carnotite. Every line was scrutinized between the range 4000 and 7600, especial pains being taken in the red and yellow portions, the least satisfactory from a photographic point of view. No discernible differences between the two spectra were observed. Because no lines were detected between wave-lengths 7800 and 2200 in any of the samples which were not due either to ordinary lead or to unimportant traces of well-known impurities, one of the alterna-

CHEMISTRY. RICHARDS AND WADSWORTH

699

tive conclusions previously reached by Richards and Lembert is supported, namely, that this isotopic lead possesses the same spectrum as ordinary lead. The present outcome is especially interesting.because the isotopic lead obtained from the preparations of Dr. Gleditsch was probably almost pure-certainly much purer than that examined in 1913. Since the atomic weight is variable, but the spectrum and atomic volume9 of these samples all the same, one can hardly avoid concluding that a part of the atom exercising an important effect upon the atomic weight is without influence upon the spectrum or volume. The dual nature thus postulated is, of course, in accord with the interesting hypothetical assumptions which have been advanced by various authors concerning the possible makeup of the atom; but our present research can go not further than support the idea of duality without defining exactly of what the two parts may consist. We are glad to acknowledge our indebtedness to the Carnegie Institution of Washington for generous support in this investigation. Summary.-In this paper the atomic weight of four different samples of isotopic lead not hitherto tested, as well as one sample of ordinary lead (used to control the others), was determined. The results were as

follows: 207.18 Ordinary lead 207.00 Isotopic lead (Carnotite, Colorado) ............................... Isotopic lead (Carotite, Australia) ................................... 206.34 206.12 Isotopic lead (Broggerite, Norway) 206.08 Isotopic lead (Cleveite, Norway) That the most carefully selected sample should give the lowest result evidence that the higher isistrong (although not absolutely conclusive) results obtained from other samples were due merely to the accidental admixture of ordinary lead. No new lines were found either in the ultraviolet or visible spectrum of any of these samples. Each, except the ordinary lead, possesses radioactivity, but the magmtude of this radioactivity seemed to bear no relation to the lowering of the atomic weight. Richards and Lembert, J. Amer. Chem. Soc., 36, 1329 (1914); Hdnigschmid and St. Horovitz, Paris, C. R. Acad. Sci., 158, 1798 (1914); M. Curie, Ibid., 158, 1676 (1914); Soddy and Hyman, London, J. Chem. Soc., 105, 1402 (1914); also especially, HSnigschmid, Sitz. k. Akad. Wiss., Wien, Ha (Dec., 1914). 2A description of the details is to be found in the paper by S. Radcliff, J. Proc. R. Soc., New South Wales, 47, 145 (1913). J. Amer. Chem. Soc., 38, 223; these PROCEEDINGS, 2, 505 (1916). 'Loc. cit., these PROCEEDINGS, 2, 505 (1916). 6 A fuller description is given in our paper published in the December, 1916, number of ....................................................

...

....................................

.....................................

J. Amer. Chem. Soc.

700

PALEONTOLOGY: W. H. DALL

' Richards

and Wadsworth, J. Amer. Chem. Soc., 38, 1659 (1916). Baxter and Grover, J. Amer. Chem. Soc., 37, 1027 (1915). a H6nigschmid and St. Horovitz, Sitz. k. Akad. Wiss., Wien, 123, 1 (2407) (Dec., 1914). He gives the name 'Uranblei' to this form of lead. The name is appropriate if, as seems probable, the substance may ultimately be traced back genetically to uranium, according to Boltwood's brilliant and well-supported hypothesis. Because the relation of this form of lead to radium is somewhat less remote than that to uranium, we used the term 'radiolead' for it in a previous paper; but this term has also been applied to Radium D and is, therefore, not distinctive. The time for a final nomenclature of these substances has probably not yet come, but the expression "isotopic lead," based upon Soddy's word 'isotope,' is certainly safe as applied to every substance fitting into this place in the Periodic System. We venture to suggest that the present permanent isotope of lead be called 'isolead,' because it is now by far the best known of these isotopes. If any other permanent leads are verified, they might be called 'meta' and 'para.' For the highly transitory isotopes these names would be inappropriate, since they do not resemble lead in one of its most characteristic properties, namely, permanence. These might be called 'pseudo-leads,' giving them Greek ordinal prefixes to distinguish between them. But we offer these suggestions without any desire to be insistent, and have not even adopted this nomenclature in the present paper. It may well be best to retain the present nomenclature, especially as regards the

IIa,

transitory isotopes.

' See Richards and Wadsworth, these PROCEEDINGS, 2, 505

(1916).

ON SOME ANOMALIES IN GEOGRAPHIC DISTRIBUTION OF PACIFIC COAST MOLLUSCA By William Healey Dall SMITSONIAN INSTITUTION. WASHINGTON, D. C. Recived by te Academy. November 9.1916

The islands, usually called the Santa Barbara Islands, lie off the coast of southern California from 15 to 50 miles roughly parallel to the shore of the mainland, from which they are separated by a depth of about 400 fathoms. They range in latitude from 33° to 34°N. That they were formerly connected with the mainland seems probable from general considerations, as well as the fact that the fossil tooth of an elephant is reported to have been found on Santa Catalina. As we go south we find other islands or banks farther off shore and separated from the mainland by an increasing depth averaging more than 1500 fathoms. The Cortez Bank lies about 120 geographical miles south of Santa Cruz Island of the Santa Barbara group, in latitude 32° 20'N. It is 80 miles west of the mainland and separated by a depth of 1090 fathoms. Next comes Guadelupe Island in latitude 29° N., 200 miles south of the Santa Barbara islands and 150 westward from the nearest Lower California mainland, from which it is separated by a depth of 1500 fathoms.